What's the most efficient way to earn credits in Puyo Puyo Tetris (ぷよぷよテトリス)?

In Puyo Puyo Tetris (ぷよぷよテトリス) playing games in the various modes earns you credits. These credits can then be used to unlock things like skins and characters. However, after playing for 5-ish hours (mostly single-player Adventure, and a mix of the multiplayer modes), I barely have 2000 credits earned. This is nowhere near enough to unlock everything, it's roughly enough to unlock a character or two. I'd like to do this more efficiently, just so everything's available.
What's the most time efficient way to earn credits in Puyo Puyo Tetris?
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